1. Field of the Invention
The present invention relates to low-density composite articles and, in particular, to low-density rocket nozzle components. The present invention further relates to a process for making the low-density composite rocket nozzle components.
2. State of the Art
Solid rocket motor nozzle components have been fabricated using conventional composite starting materials referred to as pre-pregs. Pre-preg materials generally include fabric and/or fiber that has/have been pre-impregnated with resin, typically a phenolic resin. The fabric or fiber is referred to as the reinforcement of the composite while the resin is called the composite matrix or matrix formulation.
Depending upon the position and function of the component in the nozzle and the intended application of the nozzle, either a standard-density or low-density material (pre-preg) may be used.
Historically, pre-pregs for fabricating standard-density composite rocket nozzle components include the reinforcement, matrix formulation, and appropriate fillers. In the case of standard-density carbon or graphite cloth reinforcement and phenolic resin, carbon having substantially the same density as the carbon fiber is selected as the filler. Carbon or graphite fibers can be rayon, polyacrylonitrile (xe2x80x9cPANxe2x80x9d) or pitch-based materials. Glass and silica composite pre-pregs utilize silica fillers when fillers are used.
To achieve a low-density composite (LDC) pre-preg, which is advantageous in reducing motor weight of a rocket motor formed therefrom, hollow spheres, such as described in U.S. Pat. Nos. 4,268,320, 4,294,750, or U.S. Pat. No. 4,621,024, are introduced into the pre-preg formulation as the filler. The effective densities of these hollow spheres typically range from 0.2 g/ml to 0.5 g/ml. To prevent the hollow spheres from clumping in the pre-preg, an elastomer is added to the resin mix to maintain a more even dispersion of the hollow spheres during pre-impregnation of the fiber/fiber reinforcement. However, due to the expense associated with hollow spheres and the elastomer added to the resin mix, as well as other known difficulties in producing the low-density pre-preg, the cost of the conventional low-density composite material can be 50 to 100 percent higher than that of the standard-density version of the material.
The inclusion of hollow spheres and elastomer in the pre-preg formulation also results in a composite having an across-ply tensile strength as low as one-tenth that of standard-density material. The lower across-ply tensile strength of LDCs significantly increases the likelihood of the LDC rocket nozzle components experiencing ply lifting, wedge outs and other failure phenomena. LDCs used in exit cone environments can exhibit ply lift. The tendency of these materials to exhibit these failure modes must be addressed and accommodated by nozzle design. Such accommodation typically involves making the components thicker to improve margins of safety; however, the added thickness of the components partially offsets the weight advantage, i.e., the lower density, that LDC materials have over standard-density materials.
One predominantly used process for the fabrication of conventional so-called standard-density nozzle components involves applying material to a mandrel such as by tape wrapping; ply-by-ply applying and debulking of pre-preg at very high pressures and temperatures to soften the resin, immediately followed by cooling; and autoclaving or hydroclaving curing, such as by pressurized curing at 200 to 1000 psig. The material is applied to the mandrel in such a way as to achieve 80 to 95 percent of the material debulk (compaction) required in the final component. Currently practiced processes can require a pressure greater than 800 psig to 2400 psig, to achieve desired debulking. Final debulking is achieved during the pressurized cure. This process provides a cured composite specific gravity (SpG) in carbon/graphite phenolic components of 1.40 to 1.60, glass phenolic components of 1.95 to 2.05 and silica phenolic components of 1.70 to 1.80 (g/ml).
The conventional pre-pregs are designed to be used at elevated (high) pressures and temperatures to produce fully densified composites.
Each of the above-mentioned processes have different drawbacks, some of which are noted above. In particular, the art has sought low-density composite rocket nozzle components which are produced at lower average per unit cost, but which are capable of exhibiting the erosion resistance, charring resistance, and across-ply tensile strengths of a comparable rocket nozzle component made by the conventional process with a standard-density composite.
The present invention provides a process which overcomes the above-mentioned and other drawbacks associated with the conventional manufacture of composite rocket nozzle components and addresses the needs expressed above, while affording a reduction in manufacturing costs.
The present invention also provides articles, such as composite rocket nozzle components, that synergistically combine the excellent physical properties and low production costs of standard-density nozzle components with the reduced weights of LDC nozzle components, even when the components are substantially or completely devoid of low-density fillers (microballoons etc.).
The present process achieves the aforementioned advantages while enabling the practitioner to avoid the need to use a specially designed pre-preg to fabricate low-density composite articles, including rocket nozzle components.
These and other features and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the present invention.